Asthma is a progressive inflammatory airways disease that leads to structural airway changes and debilitating symptoms in many severely affected adults. We need novel therapeutic agents that are affordable, can decrease the reliance on steroids, and can improve quality of life. This clinical and mechanistic study has the potential to impact treatment of a subset of adult severe asthmatics and to further our understanding of the mechanisms of L-arginine metabolism and NO biology in the airways of asthmatics. We will pursue a clinical trial in subjects not well controlled on standard drug therapy; this strategy will address whether L-arginine is efficacious in patients receiving standard of care medications. In studies using animal models, we and others have shown that interventions that augment NO levels, through either supplementation of L-arginine or inhibition of arginase, decrease allergic airway inflammation and hyperresponsiveness-the two hallmarks of asthma. Overall, we hypothesize that a responder subset of adult severe asthma patients will derive clinical benefit from supplemental L-arginine therapy and that these patients will have a lower exhaled NO concentrations (<20 ppb) and a higher NOS2/Arg1 mRNA and protein ratio in their airway epithelial cells than non-responders. We aim to: 1) test the hypothesis that uncontrolled, adult severe asthma patients with exhaled breath NO concentrations <20 ppb will have fewer asthma exacerbations over 3 months when treated with L-arginine compared to patients with FeNO > 25, 2) determine the mechanisms by which L-arginine affects the regulation of NOS and arginase enzymes in primary airway epithelial cell cultures from severe asthmatic subjects, and 3) test the hypothesis that inhaled nanoparticle carrier formulations of L-arginine will decrease airway inflammation, airway hyperresponsiveness, and airway fibrosis at lower doses than systemically administered L-arginine. The major impact of our study will be to identify the adult severe asthma cohort that will benefit from supplemental L-arginine therapy. Our ultimate goal is to develop novel therapeutic agents to treat adult severe asthma patients better.